The receptor NK.sub.2 of tachykinins is widely expressed in the peripheral nervous system of Mammalia. One of the several effects caused by the selective stimulation of the receptor NK.sub.2 is the contraction of the smooth muscles. Therefore, antagonists of the receptor NK.sub.2 can be considered agents able to control the hypercontraction of the smooth muscles in any patological condition in which the release of the tachykinins contributes to the rise of the corrispondent disorder. In particular, the bronchospastic component of asthma, cough, pulmonary irritations and local spasms of the urinary bladder and of the ureter during cystitis, infections and renal colics can be considered conditions in which the administration of receptor NK.sub.2 antagonists can be effective (A. L. Magnan et al. Neuropeptides, 1993, 24, 199). Compounds which act as antagonists of the tachykinins, and in particular of the neurokinin A, are well-known in Literature. Among them, the cyclic compounds (B. J. Williams et al. J. Med. Chem., 1993, 36, 2) are of particular interest. Lipophily has been defined as an essential requirement in order to have an intensive antagonist activity to the receptor NK.sub.2 of the tachykinins of a series of cyclic pseudopeptides (L. Quartara et al. J. Med. Chem., 1994, 27) and particularly in case of bicyclic hexapeptides. WO/93/21227). Surprisingly it has been now found that products structurally similar to those described above, but in which, however, at least one hydrophilic group is present, not only keep their high affinity in vitro, but also show an increase in the pharmacological activity in vivo if compared to the corrispondent compounds which do not contain any hydrophilic group.
This is even more surprising if it is taken into account that monocyclic peptides having antagonist properties which are similar to those of the tachykinins do not show any increase in the pharmacological activity when hydrophilic groups are introduced onto the structure of the cycle [Int. J. Peptide Protein Res. (1984), 44:2, 105-111].